Most participants in he Reverse Beacon Network currently use Software Research Labs' QS1R receivers for multi-band simultaneous monitoring. However, both cost and availability issues now have us looking for another option. I am one of the RBN organizers, and also manage its gap-filler program, which aims to get RBN nodes going in under-served areas, particularly along the equator and in the developing world. So cost is important to us.

From experiments that some friends have done, Pavel Demin's software appears to work very well for interfacing Skimmer Server with the Red Pitatya, covering up to 6 x 192 KHz slices. However, I am interested in learning whether anyone has professionally measured the input impedance and sensitivity of the Red Pitaya when used as an SDR. One source reports that the input impedance varies widely from 3-30 MHz, while another suggests adding a 50-ohm load across the inpuit, and making up the loss with a 50-ohm preamp of some sort. We are already providing a preamp in our gap-filler configuration, so maybe this would not be a difficult thing to do, but simplicity is also nice.

wideband preamplifier between antenna and the Red Pitaya input (with or without terminating/load resistors)

I'd be also very interested to see more measurements of the input impedance and sensitivity as well as a comparison of the various impedance matching setups. The results that I've seen so far are summarized in my yesterday post on the HamSCI mailing list.

Comparing figures the sensitivity (14 MHz) of the RP is - 106 dBm. With an easy modification you get a gain of about 6-7 dB. That adds up to -113 dBm. When I add 3 dB (my impression) of the skimmer digging in the noise you will end with around 116 dBm.

But this is not my point.

I ran parallel a Kx3 (-123 dBm)with a Px3 on a poor EndFed antenna between two houses. Of course I wanted to catch the good guys and had a hard time with CW and QRP. The reported SNR from skimmer was comparable(equal) to the Signals on the Kx3. Reported signals between 1-5 SNR were only to find with CAT and waterfall and I had nearly no chance to work them.
Now I use an active broadband CPOL antenna indoors. The loss of about 3-6 dB is equivalent to real signals on the EndFed.

Beside that an impedance matching for receivers seems to me quite academical, I have the practical impression that the sensitivity of the RP is more than sufficent. More would make reports coming from RBN-monitoring stations even harder to compare or evaluate. (I was just thinking about stations with 6*6*yagis )

Beside that an additional amp connected to a big antenna could drive the AD-Converter of the RP into saturation and clipping could be a problem.

Yes I have done recepetions on all bands (except 160m, 60m and 6m) and compared them with the Kx3. All signals had comparable S-Meter peaks. Of course signals on 80 and 40 were more hidden in local and other noise, but when the RP/Skimmer reported a signal it was visible also in the waterfall. I found no other additional decodable signals on the Kx3.

I had more problems on the higher bands 10m, where sometimes reports came, which I could not seen on the Kx3.

Hi Hajo - and thanks for the quick reply. Am I correct in assuming that this was MDS with no antenna connected, or with a dummy load? If so, -106 dBm is not very good. I have a 20M *band noise* floor of -120 dBm, so a -106 dBm internal limit would cost me many weak signals that I could otherwise hear. Of course, we could add a preamplifier to bring band noise above the internal limit.

Please I do have some problem with my RP as a skimmer at OL7M. I have tried a few images - thanks Keith W4KAZ - but still the same.
I can run RP with Peter's CWSL as CW + RTTY skimmer and also Winrad. Also if I run just CW skimmer directly, the problem is still there.
What is going on: skimmer works great ( using 4 bands / 96kHz ) but after abt 20-30hours the skimmer SW shows decoders, but nothing decoded.
I looked into the stream (is still the same on LAN) and there is empty band only with some "pings"
There is the screen: http://ok2zaw.com/SDR/skimmer.png

I have tried 3 PC, different LAN switch, RP images - BUT still last SDR RX from RP repository...